Chronic HCV infection affects approximately 2% of the US population. An improved understanding of virus-host interactions and the factors leading to viral clearance are essential in the development of improved therapies. This proposal directly addresses these needs. The first specific aim is to employ HCV replicon systems to define the requirements for a permissive host environment for replication. The hypothesis of this section is that a specific cellular environment is required for HCV RNA replication and that identification and utilization of cells with different capacities for replication provides a mechanism to approach the definition of factors that influence replication. The host-virus interaction will be explored through the use of a variety species of primary hepatocytes and livered derived cell lines. Gene array technology will be employed to define genes that positively or negatively regulate HCV RNA replication. The second specific aim is to utilize gene array technology to examine chimpanzee liver tissues at various stages of HCV infection to define changes in liver gene expression. The hypothesis of this aim is that specific alterations in liver gene expression will accompany different defined stages of infection with HCV. Changes will represent direct alterations in the hepatocyte due to interaction with HCV gene products; alterations in hepatocyte gene expression in response to the inflammatory response; and alterations in immune response genes will occur and change as the infection progresses. The later changes may be predictive of whether the infection results in viral clearance or persistence. The third specific aim is to defined factors relate to viral clearance. The emphasis will be the evaluation of cytokine and chemokine mRNA profiles in the liver of chimpanzees during well-defined phases of infection. The hypothesis is that specific cytokine profiles will be predictive of whether the animal clears HCV. We will utilize quantitative RT-PCR and a specialized gene array for cytokines, chemokines and their receptors. This approach will provide a more holistic picture of the immune response to HCV infection and the type of response associated with viral clearance. Cytokines that are candidates for potentially modulating HCV replication will be tested for antiviral activity.